Internal Combustion Engine Configurations
Internal combustion engines have wide applicability in both mobile and stationary power production applications. The most common type of internal combustion engine is a crank driven reciprocating piston engine. This type of engine includes a cylinder with a moveable piston position therein, and defines a combustion chamber between a closed end of the cylinder and the piston. A rod interconnects the piston with an offset journal on a rotateable crankshaft such that rotation of the crankshaft causes the piston to reciprocate upwardly and downwardly within the cylinder. While traditional crank driven engines are the most common, numerous other engine configurations have been proposed and used. One example is the Wankel rotary engine wherein a lobed rotor rotates within a housing to create expanding and contracting combustion chambers.
Another internal combustion engine configuration is shown in FIG. 1. This engine configuration has gone by various names, including barrel engine, axial engine, axial piston or cylinder engine, cam engine, swash ring or plate engine, crank plate engine, cam or wave cam engine, wobble plate engine, and radial or rotary engine, among others. For purposes of the present application, these types of engines will be referred to as barrel engines. However, it should be understood that the term “barrel engine,” as used herein, is not limited to the specific configurations illustrated, but instead refers to similar designs as well.
The engine 10 in FIG. 1 is merely representative of the general configuration of the engine referred to herein as a barrel engine. It includes a crankshaft or power shaft 12 with a plurality of cylinders arranged about the power shaft 12, though single cylinder variations are possible. The central axis of each of the cylinders 14 may be generally parallel to the power shaft 12. Alternatively, the axes of the cylinders 14 may be tilted outwardly or inwardly with respect to the power shaft 12. A cam plate or track 16 is preferably connected to the power shaft 12 such that the two rotate in unison. The track 16 surrounds and extends outwardly from the power shaft 12 and has an undulating cam surface 18. As the power shaft 12 is rotated about its longitudinal axis, the cam surface 18 of the track 16 undulates closer to and farther from the cylinders 14. Pistons 20 are moveably positioned in the cylinders 14 and define a combustion chamber 22 between each piston and the upper end of its respective cylinder 14. The pistons 20 are interconnected with the track 16 such that as the track rotates, the pistons are caused to reciprocate within the cylinders 14. In the illustrated embodiment, connecting rods 24 have upper ends interconnected with the pistons 20 and lower ends with rollers 26 that ride on the cam surface of the track 16. Alternatively, pistons in such an engine may be more directly interconnected with the track, such as rollers or slides directly connected to the pistons.
As will be clear to those of skill in this art, as the power shaft 12 rotates and the pistons 20 reciprocate within their respective cylinders, the various strokes of a combustion cycle can be defined. Typically, the cam surface 18 of the track 16 has a generally sinusoidal shape, thereby corresponding to the reciprocal motion typical of a crank driven piston. Also, the track is generally disposed in a plane perpendicular to the power shaft and the cam surface is generally disposed at a constant distance from the axis of the power shaft.
Barrel engines maybe either single ended or double ended. In a single ended design, cylinders and pistons are provided in the end of the engine on one side of the track, such as above the track as illustrated in FIG. 1. In double ended designs, cylinders and pistons are provided on both ends of the engine (both above and below the track when positioned as shown in FIG. 1). Another variation includes a track at both ends of the engine and opposed pistons extending towards one another from the two tracks. The tracks typically rotate in unison causing two pistons to reciprocate towards and away from one another within a common cylinder.
Applicant's prior applications, referred to in the Reference to Related Applications, and incorporated herein by reference, discuss various engine designs generally referred to as Inverse Peristaltic Engines. Applicant considers barrel engines to be a variation within the general class of Inverse Peristaltic Engines, as described in these applications. Other variations on Inverse Peristaltic Engines share certain functional attributes with barrel engines as described within the present application. It should be noted that some aspects of the present invention may be used with engine configurations other than the particular configurations illustrated or described.
Another engine design that has some functional and/or structural similarities to barrel engines, as thus far described, is a type of engine often referred to as a wobble plate engine. FIG. 2A illustrates a schematic of a portion of a wobble plate engine 30. In the barrel engine 10, the track 16 is illustrated as having two low points and two high points corresponding to a complete set of four strokes for a combustion engine. In the wobble plate engine, a plate 32 is interconnected with the longitudinal powershaft 34. The plate 32 is generally planar, but is angled with respect to the powershaft 34 such that it has high and low portions. That is, rather than the 32 being perpendicular to the shaft 34, it is tilted somewhat. Pistons 36 and 38 are in mechanical communication with the plate 32, in a manner similar to in FIG. 1. However, because the plate 32 is generally planar instead of being more complexly shaped, such as the track 16, only two strokes are defined within a single rotation of the plate 32. That is, if the shaft 34 is rotated through one complete revolution, each of the pistons 36 and 38 would experience only a single top-dead center and bottom-dead center. With the barrel engine of FIG. 1, on the other hand, a single rotation of the track 12 causes each of the pistons 20 to travel to top-dead center twice and bottom-dead center twice. Designs similar to the engine 30 have been used for wobble plate compressors. In these compressors, the angle of the plate may be adjusted to adjust the compression ratio of the compressor. Typically, such a compressor has spherical rollers interconnecting the pistons with the plate. An engine may be constructed similarly. The design of FIG. 2A and compressor-like variations are considered to be barrel engines, as defined herein.
Referring now to FIG. 2B, another version of an engine 40 is illustrated. In this engine 40 a shorter longitudinal powershaft 42 connects with an angled cam 44, that is generally triangular in cross-section. The powershaft 42 and angled cam 44 rotate in unison. A wobble plate or piston support plate 46 rides on the upper surface of the angled cam 44, but does not rotate therewith. Therefore, as the angled cam 44 rotates, the piston support plate 46 tilts back and forth. Pistons are interconnected with the piston support plate 46 such that movement of the plate 46 causes reciprocal motion of the pistons. This is again considered a type of barrel engine as defined herein. It differs from the two previous designs in that the piston support plate 46 replaces the rollers for communicating movement between the cam 44 and the pistons.
Other versions of barrel engines, as defined herein, include a type of engine called nutating engine. An example is shown in U.S. Pat. No. 5,992,357 which is incorporated in its entirety herein by reference. Another barrel engine design, sometimes referred to as a bent-axis engine, is shown in U.S. Pat. No. 1,293,733 to Duby, which is also incorporated herein in its entirety by reference.
Combustion Strategies for Internal Combustion Engines
A variety of combustion strategies have been proposed and/or tested for internal combustion engines. The most common strategy, referred to herein as spark ignition (SI), is illustrated in FIG. 3. Air and fuel are mixed together prior to being drawn into a combustion chamber. The mixture is compressed in the combustion chamber and a spark is then provided to ignite the compressed mixture. This process is used in most gasoline-fueled internal combustion engines. Spark ignited internal combustion engines include both two stroke and four stroke reciprocating piston designs, as well as some less well known varieties. The air and fuel is typically mixed upstream of the cylinder using a carburetor or fuel injectors. The air and fuel mixture for multiple cylinders may be created at a single point, such as with typical carburetors and throttle body fuel injection systems, or fuel and air may be mixed individually for individual cylinders, such as occurs with port fuel injection. A less common approach is to directly inject fuel into the cylinder prior to or during the compression stroke. Whatever the variation, spark ignition engines are characterized by the fact that combustion is initiated by the introduction of a spark to a compressed air and fuel mixture. Spark ignited engines have the benefit that combustion timing, also referred to as combustion phasing, is easily controlled. Because combustion is initiated by the introduction of a spark, combustion timing can be controlled by controlling spark timing. Spark ignition engines also tend to be relative compact and less expensive than some other types of engines. A drawback to spark ignition engines is lower fuel efficiencies than some other types.
Another well known combustion strategy is the approach used with Diesel engines, illustrated in FIG. 4. In a Diesel, air, without fuel, is drawn into a combustion chamber and compressed. Once the air is partly or completely compressed, fuel, typically Diesel fuel, is injected into the compressed air. The introduction of the fuel into the compressed air, under the appropriate conditions, causes the fuel and air mixture to combust. Variations on the diesel strategy include the introduction of fuel at more than one stage in so called stratified-charge Diesel engines. In a stratified-charge Diesel engine, the air fuel mixture is intentionally manipulated to create areas of richer and leaner fuel concentrations. Often this is accomplished by compressing an initially lean mixture and then adding additional fuel to create localized rich areas and to initiate combustion. Stratified-charge or lean combustion approaches have also been used with spark ignition engines. Combustion phasing is also easily controlled in a diesel engine, since fuel injection timing determines combustion timing. Diesel engines offer improved fuel efficiency in comparison to spark ignited engines, and offer the ability to combust less expensive types of fuels. However, Diesel engines tend to be heavier, more expensive and noisier than spark ignited engines. Also, diesels produce high levels of oxides of nitrogen (NOx) and particulate emissions.
Another combustion strategy, referred to herein as homogenous charge compression ignition (HCCI), is illustrated in FIG. 5. In HCCI, a mixture of air and fuel is drawn into a combustion cylinder. The mixture is then compressed until the mixture autoignites, without the introduction of a spark. Variations on HCCI include injection of fuel directly into the cylinder at some point during the compression stroke so as to promote a substantially premixed charge. The HCCI combustion strategy has been referred to by various names, including controlled auto-ignition combustion (Ford), premixed charged compression ignition (Toyota and VW), active radical combustion (Honda), fluid dynamically controlled combustion (French Petroleum Institute), and active thermo combustion (Nippon Engines).
HCCI offers several benefits over spark ignition and Diesel strategies. First, HCCI offers the potential for significantly increased fuel efficiency. Second, emissions from HCCI are more manageable than for other strategies. HCCI combustion is significantly cooler than conventional combustion and therefore has significantly lower NOx emissions. HCCI also produces less particulate emissions than a diesel engine. Additionally, the absence of locally rich regions found in conventional Diesel engines reduces or eliminates particulate emissions and smoke. The benefits and drawbacks to HCCI, as well as strategies for controlling HCCI, are more extensively discussed in SAE paper no. 1999-01-3682, which is incorporated herein in its entirety by reference.
A drawback to HCCI is that combustion phasing is very difficult to control. The autoignition point of a compressed mixture of air and fuel depends on numerous factors, including the exact makeup of the fuel, the temperature of the mixture, the temperature of the cylinder, the makeup and reactivity of any other components present in the combustion chamber, the shape of the combustion chamber, the operating speed of the engine, the operating load of the engine, and numerous other factors. There has thus far been no practical way to effectively control HCCI combustion in an engine subject to normal transients in load and RPM. Unlike diesel and spark-ignited engines, where the phasing of combustion can be controlled by timing when fuel is injected or when a spark is introduced, HCCI engines lack a direct method of controlling the start of combustion.
Another challenge with HCCI is related to combustion rate. Combustion in HCCI engines occurs at multiple ignition points within the combustion chamber and unlike diesel ignition, in which the rate of combustion is controlled by the mixing rate of the fuel jet and oxidizer, pressure rise in HCCI can occur at an extremely rapid and destructive rate unless very lean air-fuel mixtures are used. The requirement for lean air-fuel mixtures limits the maximum power output of HCCI engines to 50–75% of that of equivalent diesel and Otto cycle engines, placing limitations on the markets in which HCCI engines can be used.
Control Strategies for HCCI
Numerous approaches have been proposed for controlling combustion phasing in an HCCI engine. One approach to controlling the combustion phasing of an HCCI engine is to adjust the compression ratio of the engine. The mixture of the air and fuel will autoignite once it is sufficiently compressed. However, the amount of compression necessary to initiate combustion depends on numerous factors. By varying compression ratio, combustion phasing can be controlled. Higher compression ratios result in earlier combustion and lower compression ratios result in later combustion, or a lack of combustion. Several variable compression ratio engine designs have been proposed, and in some cases, built. These engines suffer from mechanical complexity and increased costs. Additionally, depending on the method used to vary the compression ratio in the engine, changes cannot be made quickly enough to adequately control combustion phasing in an HCCI engine. Also, some designs restrict the placement of valves and create crevice areas in the combustion chamber, thereby leading to lowered efficiency and increased emissions. Such a design is disclosed in SAE paper 1999-01-3679, which is incorporated herein by reference.
Another method for controlling combustion phasing in HCCI engines is to control the temperature of the intake air. As the intake air temperature is increased, with all other conditions held constant, combustion will occur earlier. Reducing the temperature of the intake air delays combustion. Therefore, by controlling the intake air temperature, combustion phasing may be controlled to some extent. Drawbacks to this approach include reductions in volumetric efficiency as intake air temperature is increased and complications related to the provision of heated intake air. Precise control of the intake air temperature at the combustion chamber is also difficult, and the range of adjustment available with this approach is quite limited.
Fuel blending is an additional method for controlling HCCI combustion phasing. Different types of fuels autoignite under different conditions. Therefore, by blending two or more fuels with different propensities to autoignite, combustion phasing can be adjusted. This approach is typically limited to stationary applications. Obvious drawbacks include complications associated with redundant fuel systems and the need for an infrastructure to support distribution of disparate and exotic fuels.
SAE Paper 2000-01-0251 (incorporated herein by reference) discusses the use of residual exhaust gas as a method of controlling HCCI combustion phasing. As the amount of exhaust gas introduced to the combustion chamber is increased, combustion occurs earlier. Drawbacks to this approach included a limited range of control, reduced power and efficiency at high residual levels, and the requirement for high residual levels under certain conditions.
U.S. Pat. Nos. 5,832,880 and 5,875,743 to Dickey propose the use of water injection to control combustion phasing. Water is introduced either in the intake manifold or directly into the combustion chamber. The introduction of water into the combustible mixture delays the onset of combustion. This approach requires the provision of a very controllable water injection system and there is some concern that the injection of water into the combustible mixture may increase engine wear. Also, this approach has not provided adequate control according to researchers in the field.
Yet another approach to HCCI combustion phasing control is proposed in U.S. Pat. No. 6,260,520 to Van Reatherford (incorporated herein by reference). This patent proposes providing a secondary compression device designed to provide additional compression of the mixture in the combustion chamber. In this patent, a secondary boost piston is provided in the cylinder head such that movement of the piston increases and decreases the combustion chamber volume. In operation, the mixture of air and fuel is first compressed by the primary piston. Then, the secondary piston is moved to further increase the compression in the combustion chamber until the mixture autoignites. The timing of the movement of the secondary piston controls the onset of combustion, thereby allowing control of combustion phasing. This design is mechanically complex and increases the crevice volume in the combustion chamber.
Variable valve timing has also been proposed as a method of controlling combustion phasing. By controlling the valve timing of an engine, the effective compression ratio can be somewhat modified.
Despite substantial effort by numerous parties, no control strategy has proven particularly effective at regulating HCCI combustion phasing. This is particularly true where the HCCI engine would experience fast changes in speed and load.